In recent years, attention has been given to hybrid vehicles and electric vehicles against the background of environmental issues. These vehicles are equipped with an electric motor as a drive power source, and are also equipped with, e.g., a secondary battery as its electric power source. In general, the secondary battery mounted on the vehicle includes a plurality of battery blocks connected in series together. Each battery block includes a plurality of battery cells connected in series together.
For example, Japanese Patent Laying-Open No. 2002-101565 has disclosed a voltage adjusting device and method that can adjust variations in terminal voltage between the plurality of unit cells forming a battery assembly. This voltage adjusting device includes upper limit voltage sensing means for sensing that the terminal voltage of each unit cell exceeds an upper limit voltage, discharging means configured to discharge the unit cell of which terminal voltage sensed by the upper limit voltage sensing means exceeds the upper limit voltage, voltage sensing means for sensing a voltage of a cell group formed of some of the units cells forming the battery assembly and charge/discharge control means controlling the charge/discharge performed between the battery assembly and a load. When the upper limit voltage sensing means senses the unit cell having the terminal voltage exceeding the upper limit voltage, the charge/discharge control means controls driving of the load such that the discharge from the battery assembly is performed preferentially. Further, based on a result of the sensing by the upper limit voltage sensing means and an average unit cell voltage of the cell group obtained by the voltage sensing means, the charge/discharge control means may determine that the variations in terminal voltage between the unit cells forming the cell group in question have increased to a predetermined level or higher, in which case the charge/discharge control means performs the charging such that the average unit cell voltage of this cell group exceeds the upper limit voltage by a predetermined voltage.
When the above control method is used, however, the cell group is charged such that the average unit cell voltage exceeds the upper limit voltage by the predetermined voltage so that characteristics of the cell group may deteriorate. Japanese Patent Laying-Open No. 2002-101565 has not disclosed a possibility of occurrence of this problem.